1. Field of the Invention
This invention relates to X-ray photographic devices.
2. Prior Art
X-ray photographic apparatus of the non-integrated type where the X-ray tube module is substantially independent from the target module are known to the art. In diagnostic radiology apparatus of this type is often employed for producing X-ray photographs of bedridden patients. In such instances, when the patient cannot be taken to a large fixed bed X-ray machine, mobile mounted X-ray apparatus are brought to the patient. Independent film cassettes are arranged beneath or next to the patient and the X-ray beam is directed at the target cassette. Such devices normally include a wheel mounted member equipped with articulated supports or arms to which the X-ray tube is secured. The tube may therefore be simply adjusted to any desired position such that the beam will be directed relative to the patient and the film cassette.
In the case of medical X-ray image photographs, it is desirable to produce both the best possible image sharpness and contrast while exposing the patient to the least possible radiation dosage. It is also desirable to limit the dosage for protection of the personnel conducting the examination.
In practice, in order to achieve the desired image result, the beam is restricted by a diaphragm associated with the X-ray tube housing. This restriction of the diaphragm aperture is of major significance. Among other things, the X-ray photograph image impairing effect produced by secondary radiation emitting from the patient and which results in background veiling or ghost imaging is reduced by diaphragm limitation. However proper adjustment of the diaphragm is difficult to realize in those instances where a mechanical linkage or coupling between the X-ray tube and the film cassette is not present. The obvious and most often practiced process is to align the X-ray beam in relation to the film cassette by eyeballing the relative disposition of the X-ray tube housing, the primary diaphragm and the film cassette. A major disadvantage with such eyeballing is that if the X-ray beam is too narrowly limited by the diaphragm, the desired image is only partially obtained. In order to avoid this it is common practice to leave the diaphragm setting wider than is necessary for use in connection with the particular cassette employed, thereby resulting in an unnecessar large radiation dose.
Additionally, good stopping-down of the X-ray beam by the diaphragm is of major significance in the reduction of the charge or burden of the overall radiation dose on the patient. As a result, the appropriate government agency in the United States has required automatic diaphragm limitation of the X-ray beam to the format of the film or cassette. Yet due to technical difficulties involved, an exception to this requirement is made for devices of the type in which a mechanical coupling or linkage between the X-ray tube and the film cassette does not exist. Thus, an exception from the automatic diaphragm rule specifically applies to the apparatus of the type above described. However, it would obviously be desirable to provide for such precise limiting of the beam in such apparatus.
A number of attempts have been made to limit undesired radiation. One such method of making secondary radiation less disruptive of image clarity is through the use of a so-called secondary radiation or raster screen. Raster screens may be composed of lead laminae standing on edge and having a high degree of X-ray absorption. The lead laminae are surrounded by a medium with relatively low absorption. Such raster screens are arranged between the patient and the film cassette. To the extent that the surface of the raster screen is not at right angles in relation to the central ray of the X-ray beam, the image producing direct radiation will be absorbed by the laminae in addition to the laminae's absorption of the secondary radiation. This results in a so called raster effect. The raster effect is very bothersome in practice, and often, particularly in the case of bedridden patients, leads to repeated X-ray photographs having the consequence of both an increased radiation dose on the patient and an increased consumption of film.
British patent 861,550 teaches an X-ray device having an examination table providing support for the patient. The table is used in connection with apparatus mechanically coupling the X-ray tube with the table in such a manner that only limited movement of the X-ray tube may be carried out in relation to the support or patient bearing surface. A light beam aligned with the central ray of the X-ray beam is used to mark the position or orientation of the central ray in relation to the patient and in relation to a movable cassette holder or container lying beneath the support surface. The light beam is produced by one or more light sources which are mechanically coupled to the X-ray tube. This apparatus, which requires a mechanical coupling between the X-ray tube and the film cassette, allows a visual showing of the relative position of the X-ray tube and the film cassette, but only for limited allowable movements of the X-ray tube and cassette. The apparatus taught by this patent cannot be used in association with X-ray photographic apparatus of the above discussed or non-integrated type.
It would therefore be an advance in the art to provide an X-ray apparatus having independently movable X-ray beam generating apparatus and film cassette apparatus but which provides a means for precisely aligning the X-ray beam with the cassette and for properly limiting the beam spread.